1. Field of the Invention
The present invention relates to wireless communication systems, and in particular, to wireless communications system base stations that use adaptive antenna arrays to transmit signals to wireless subscribers in order to enhance system performance.
2. Description of Related Art
In interference-limited wireless communication systems, the downlink (also known as forward link, from base station to mobile station or subscriber) system capacity is approximately inversely proportional to the average transmitting power per subscriber. Therefore to achieve the maximum downlink system capacity, it is generally required to transmit as little power as necessary to each individual wireless subscriber in the system. This task can be achieved by deploying downlink power control and/or smart antenna systems.
Power control algorithms allow base stations to transmit the minimum required power to the subscriber. In wireless systems, subscribers experience different co-channel interference mainly due to different inter-cell, inter-sector, and intra-sector interference (I_inter-cell, I_inter-sector, and I_intra-sector, respectively) levels. Subscribers also experience different received signal power levels due to different radio propagation environments. Hence, the downlink signal to interference-plus-noise ratio (SINR) at different subscribers are widely distributed. In order to achieve maximum downlink system capacity, all subscribers should have the same minimum required SINR. This may be achievable through a power control system.
Smart antenna algorithms allow base stations to effectively direct the transmitted power to the subscriber. Subscribers are generally randomly located in wireless communication systems. So most of the current base stations transmit radio signals into the whole sector which usually is a 120xc2x0 sector (in 3-sector systems) or a 60xc2x0 sector (in 6-sector systems). By doing so, the whole sector is being illuminated by the radio signals from the base station no matter where the subscriber is in the sector. This means the base station is also transmitting radio signal power into directions (areas) where the transmitted power can never reach the intended subscriber. In order to reduce these unnecessary illuminations, one can transmit radio signal power into directions (areas) that can improve the subscriber""s received signal transmitted from the base station. This may be achievable through a smart antenna system.
Since the downlink capacity gain is proportional to the amount of the average transmit power level that is being reduced by power control and/or smart antenna systems, beam forming gain should not compromise power control gain and vice versa in order to achieve the maximum system capacity gain.
In accordance with the invention, beam forming on downlink traffic channel signals is performed, whereby the direction, gain, and 3-dB beam width are adjusted based on various parameters, including frame error rates, pilot power measurements at the mobile station, angle spread estimation of the reverse link signals, distance estimation between mobile station and base station, etc. Phase-matching between the traffic channel and pilot channel signals is also performed. Phase-matching is accomplished by minimizing the average of the phase mismatch over the dominant downlink paths or by minimizing the mean-square error of the phase mismatched over the dominant paths.
Beam forming on the downlink traffic channel signals involves determining the main lobe beam direction and selecting the beam width. The direction of the downlink main lobe beam is based on an average uplink channel estimator or angle of arrival data to select the most likely direction of the desired mobile station. Once the direction is determined, the 3-dB beam width (or equivalently, the gain) is selected based on frame error rates, pilot power measurements at the mobile station, angle spread estimation of the reverse link signals, distance estimation between mobile station and base station, etc.
According to one embodiment, the narrowest width beam (or highest gain beam) is selected that has 1) a frame error rate (FER) less than a pre-defined FER threshold and 2) a traffic beam transmitting power after a beam width change less than the transmitting power prior to the beam width change. According to another embodiment, the beam width is adjusted according to the target downlink frame error rate (or target SNR), the downlink pilot signal strength, and the reverse link angle of arrival. The beam width is reduced, by increments of one step (to the next narrower beam), which is equivalent to increasing the gain (e.g., by 0.5 dB), if the mobile station is in a region where the FER reported by the mobile station is decreasing, i.e., a narrower beam can be used while still maintaining a desired FER. The beam width is increased, by increments of one step (to the next wider beam), if the FER is increasing to near the FER threshold and the pilot signal strength is increasing or unchanged. The beam width is unchanged if 1) the pilot power strength is increasing and the FER is unchanged or 2) the FER is increasing, but not to near the FER threshold. However, if there is not frame error and pilot strength reporting or if the mobile station is in a deep fading region for more than X measurement frames, i.e., the cycle of a periodic report by the mobile station, the beam width is selected to just exceed the angle spread, as estimated by AOA data or other suitable methods.
Yet another embodiment for down link beamforming selects the downlink or forward link beam width and target signal to interference-plus-noise ratio (SINRt) based on downlink pilot channel strength and downlink traffic frame errors. If a threshold number of frame errors is met, then 1) increase the beam width by a fixed number of steps if a) the beam width was decreased last time, or b) the beam width was increased last time, but not to the maximum allowable beam width and the SINRt is at the maximum value, or c) the beam width was neither changed last time nor at the maximum and the SINRt was increased last time, 2) increase SINRt by a fixed number of steps if a) the SINRt was decreased last time, or b) the SINRt was increased last time, but not to the maximum allowable SINRt value and the beam width is at the maximum value, or c) the SINRt was neither changed last time nor at the maximum value and the beam width was increased last time, and 3) leave the beam width and the SINRt unchanged if both the beam width and the SINRt are at their respective maximum allowable values.
However, if the threshold number of frame errors is not met in the last X (a predefined fixed number) measurement frames, then 1) decrease the beam width by a fixed number of steps if a) the beam width was changed last time, but not to the minimum beam size and SINRt is at the minimum allowable value or b) the beam width was neither changed last time nor at the minimum, 2) decrease SINRt by a fixed number of steps if a) the beam width was changed last time and SINRt is not at the minimum or b) the beam width was not changed last time, but is at the minimum and SINRt is not at the minimum, and 3) leave SINRt and the beam width unchanged if both SINRt and the beam width are at their respective minimum allowable values.
Regardless of the method used for downlink beam forming, a constant Effective Radiation Power (ERP) is maintained for all traffic beams in the main lobe direction for a fixed power control value. Constant ERP is maintained by changing beam forming coefficients to compensate for the different antenna gains.
The present invention will be more fully understood upon consideration of the detailed description below, taken together with the accompanying drawings.